Organic light-emitting diode display with bottom shields

ABSTRACT

A display may have an array of organic light-emitting diode display pixels. Each display pixel may have a light-emitting diode that emits light under control of a drive transistor. Each display pixel may also have control transistors for compensating and programming operations. The array of display pixels may have rows and columns. Row lines may be used to apply row control signals to rows of the display pixels. Column lines (data lines) may be used to apply display data and other signals to respective columns of display pixels. A bottom conductive shielding structure may be formed below each drive transistor. The bottom conductive shielding structure may serve to shield the drive transistor from any electric field generated from the adjacent row and column lines. The bottom conductive shielding structure may be electrically floating or coupled to a power supply line.

This application claims the benefit of provisional patent applicationNo. 61/929,907, filed Jan. 21, 2014, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to electronic devices with displays and, moreparticularly, to display driver circuitry for displays such asorganic-light-emitting diode displays.

Electronic devices often include displays. For example, cellulartelephones and portable computers include displays for presentinginformation to users.

Displays such as organic light-emitting diode displays have an array ofdisplay pixels based on light-emitting diodes. In this type of display,each display pixel includes a light-emitting diode and thin-filmtransistors for controlling application of a signal to thelight-emitting diode to produce light.

An organic light-emitting diode display pixel includes a drive thin-filmtransistor connected to a data line via an access thin-film transistor.The access transistor may have a gate terminal that receives a scansignal via a corresponding scan line. Image data on the date line can beloaded into the display pixel by asserting the scan signal to turn onthe access transistor.

In conventional organic light-emitting diode display pixels, the scanline is formed relatively close to the drive transistor. In certainoperating scenarios, the scan line may be biased in a way that ahorizontal electric field may be created between the scan line and thechannel region of the drive transistor. An electric field generated inthis way can interfere with the operation of the drive thin-filmtransistor and therefore result in undesired color artifacts.

It would therefore be desirable to be able to provide improved displayssuch as improved organic light-emitting diode displays.

SUMMARY

An electronic device may include a display having an array of displaypixels. The display pixels may be organic light-emitting diode displaypixels. Each display pixel may have an organic light-emitting diode thatemits light. A drive transistor in each display pixel may apply currentto the organic light-emitting diode in that display pixel. The drivetransistor may be characterized by a threshold voltage.

Each display pixel may have control transistors that are used incompensating the display pixels for variations in the thresholdvoltages. During compensation operations, a reference voltage may beprovided to the display pixels. The control transistors may also be usedin loading display data into the display pixels during programmingoperations and in controlling display pixel emission operations.

Each display pixel may be provided with conductive shielding structuresformed directly below the drive transistors to prevent any horizontalelectric field generated from biasing the control transistors frominterfering with the operation of the drive transistors. The conductiveshielding structures may only be formed below the drive transistors andnot the control transistors.

The conductive shielding structures may be formed from transparentconductive material or opaque conductive material. The conductiveshielding structures may be electrically floating or may be shorted to acommon power supply line such as a common cathode electrode. Inparticular, the conductive shielding structures may be formed in atleast one buffer layer interposed between the drive transistor and atransparent substrate over which the drive transistor is formed.Conductive shields formed in this way are therefore sometimes referredto as bottom shields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having adisplay in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative display such as an organiclight-emitting diode display having an array of organic light-emittingdiode display pixels in accordance with an embodiment.

FIG. 3 is a diagram of an illustrative organic light-emitting diodedisplay pixel of the type that may be used in a display in accordancewith an embodiment.

FIG. 4 is a cross-sectional side view of conventional organiclight-emitting diode display pixel structures.

FIG. 5 is a cross-sectional side view of an illustrative organiclight-emitting diode display pixel having a drive transistor and aconductive shielding structure formed directly below the drivetransistor in accordance with an embodiment.

FIG. 6 is a top view of multiple display pixels of the type shown inFIG. 5 having conductive shielding structures that are electricallyfloating in accordance with an embodiment.

FIG. 7 is a top view of multiple display pixels of the type shown inFIG. 5 having conductive shielding structures that are shorted to oneanother in accordance with an embodiment.

FIG. 8 is a diagram showing how at least some conductive shieldingstructures in a display pixel array may be shorted to a common cathodeelectrode in accordance with an embodiment.

FIG. 9 is a cross-sectional side view of a peripheral portion of thedisplay pixel array of FIG. 9 showing how the conductive shieldingstructures can be connected to the cathode electrode using vias inaccordance with an embodiment.

DETAILED DESCRIPTION

An illustrative electronic device of the type that may be provided withan organic light-emitting diode (OLED) display is shown in FIG. 1. Asshown in FIG. 1, electronic device 10 may have control circuitry 16.Control circuitry 16 may include storage and processing circuitry forsupporting the operation of device 10. The storage and processingcircuitry may include storage such as hard disk drive storage,nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory configured to form a solidstate drive), volatile memory (e.g., static or dynamicrandom-access-memory), etc. Processing circuitry in control circuitry 16may be used to control the operation of device 10. The processingcircuitry may be based on one or more microprocessors, microcontrollers,digital signal processors, baseband processors, power management units,audio codec chips, application specific integrated circuits,programmable integrated circuits, etc.

Input-output circuitry in device 10 such as input-output devices 12 maybe used to allow data to be supplied to device 10 and to allow data tobe provided from device 10 to external devices. Input-output devices 12may include buttons, joysticks, click wheels, scrolling wheels, touchpads, key pads, keyboards, microphones, speakers, tone generators,vibrators, cameras, sensors, light-emitting diodes and other statusindicators, data ports, etc. A user can control the operation of device10 by supplying commands through input-output devices 12 and may receivestatus information and other output from device 10 using the outputresources of input-output devices 12.

Input-output devices 12 may include one or more displays such as display14. Display 14 may be a touch screen display that includes a touchsensor for gathering touch input from a user or display 14 may beinsensitive to touch. A touch sensor for display 14 may be based on anarray of capacitive touch sensor electrodes, acoustic touch sensorstructures, resistive touch components, force-based touch sensorstructures, a light-based touch sensor, or other suitable touch sensorarrangements.

Control circuitry 16 may be used to run software on device 10 such asoperating system code and applications. During operation of device 10,the software running on control circuitry 16 may display images ondisplay 14 in input-output devices.

FIG. 2 shows display 14 that includes structures formed on one or morelayers such as substrate 24. Layers such as substrate 24 may be formedfrom planar rectangular layers of material such as planar glass layers.Display 14 may have an array of display pixels 22 for displaying imagesfor a user. The array of display pixels 22 may be formed from rows andcolumns of display pixel structures on substrate 24. These structuresmay include thin-film transistors such as polysilicon thin-filmtransistors, semiconducting oxide thin-film transistors, etc. There maybe any suitable number of rows and columns in the array of displaypixels 22 (e.g., ten or more, one hundred or more, or one thousand ormore).

Display driver circuitry such as display driver integrated circuit 15may be coupled to conductive paths such as metal traces on substrate 24using solder or conductive adhesive. Display driver integrated circuit15 (sometimes referred to as a timing controller chip) may containcommunications circuitry for communicating with system control circuitry16 over path 25. Path 25 may be formed from traces on a flexible printedcircuit or other cable. The control circuitry may be located on a mainlogic board in an electronic device such as a cellular telephone,computer, television, set-top box, media player, portable electronicdevice, or other electronic equipment in which display 14 is being used.During operation, the control circuitry may supply display driverintegrated circuit 15 with information on images to be displayed ondisplay 14. To display the images on display pixels 22, display driverintegrated circuit 15 may supply clock signals and other control signalsto display driver circuitry such as row driver circuitry 18 and columndriver circuitry 20. Row driver circuitry 18 and/or column drivercircuitry 20 may be formed from one or more integrated circuits and/orone or more thin-film transistor circuits.

Row driver circuitry 18 may be located on the left and right edges ofdisplay 14, on only a single edge of display 14, or elsewhere in display14. During operation, row driver circuitry 18 may provide row controlsignals on horizontal lines 28 (sometimes referred to as row lines or“scan” lines). Row driver circuitry may sometimes be referred to as scanline driver circuitry.

Column driver circuitry 20 may be used to provide data signals D fromdisplay driver integrated circuit 15 onto a plurality of correspondingvertical lines 26. Column driver circuitry 20 may sometimes be referredto as data line driver circuitry or source driver circuitry. Verticallines 26 are sometimes referred to as data lines. During compensationoperations, column driver circuitry 20 may use vertical lines 26 tosupply a reference voltage. During programming operations, display datais loaded into display pixels 22 using lines 26.

Each data line 26 is associated with a respective column of displaypixels 22. Sets of horizontal signal lines 28 run horizontally throughdisplay 14. Each set of horizontal signal lines 28 is associated with arespective row of display pixels 22. The number of horizontal signallines in each row is determined by the number of transistors in thedisplay pixels 22 that are being controlled independently by thehorizontal signal lines. Display pixels of different configurations maybe operated by different numbers of scan lines.

Row driver circuitry 18 may assert control signals such as scan signalson the row lines 28 in display 14. For example, driver circuitry 18 mayreceive clock signals and other control signals from display driverintegrated circuit 15 and may, in response to the received signals,assert scan signals and an emission signal in each row of display pixels22. Rows of display pixels 22 may be processed in sequence, withprocessing for each frame of image data starting at the top of the arrayof display pixels and ending at the bottom of the array (as an example).While the scan lines in a row are being asserted, control signals anddata signals that are provided to column driver circuitry 20 bycircuitry 16 direct circuitry 20 to demultiplex and drive associateddata signals D onto data lines 26 so that the display pixels in the rowwill be programmed with the display data appearing on the data lines D.The display pixels can then display the loaded display data.

In an organic light-emitting diode display, each display pixel containsa respective organic light-emitting diode. A schematic diagram of anillustrative organic light-emitting diode display pixel 22 is shown inFIG. 3. As shown in FIG. 3, display pixel 22 may include alight-emitting diode 30 coupled to a drive transistor TD. A positivepower supply voltage V_(DDEL) may be supplied to positive power supplyterminal 34, whereas a ground power supply voltage V_(SSEL) may besupplied to ground power supply terminal 36. The state of drivetransistor TD controls the amount of current flowing through diode 30and therefore the amount of emitted light 40 from display pixel 22.

Display pixel 22 may have storage capacitors Cst1 and Cst2 and one ormore transistors that are used as switches such as transistors SW1, SW2,and SW3. Signal EM and scan signals SCAN1 and SCAN2 are provided to arow of display pixels 22 using row lines 28. Data D is provided to acolumn of display pixels 22 via data lines 26.

Signal EM is used to control the operation of emission transistor SW3.Transistor SW1 is used to apply the voltage of data line 26 to node A,which is connected to the gate of drive transistor TD. Transistor SW2 isused to apply a direct current (DC) bias voltage Vini to node B forcircuit initialization during compensation operations.

During compensation operation, display pixels 22 are compensated forpixel-to-pixel variations such as transistor threshold voltagevariations. The compensation period includes an initialization phase anda threshold voltage generation phase. Following compensation (i.e.,after the compensation operations of the compensation period have beencompleted), data is loaded into the display pixels. The data loadingprocess, which is sometimes referred to as data programming, takes placeduring a programming period. In a color display, programming may involvedemultiplexing data and loading demultiplexed data into red, green, andblue pixels.

Following compensation and programming (i.e., after expiration of acompensation and programming period), the display pixels of the row maybe used to emit light. The period of time during which the displaypixels are being used to emit light (i.e., the time during whichlight-emitting diodes 30 emit light 40) is sometimes referred to as anemission period.

During the initialization phase, circuitry 18 asserts SCAN1 and SCAN2(i.e., SCAN1 and SCAN2 are taken high). This turns on transistors SW1and SW2 so that a reference voltage signal Vref and initializationvoltage signal Vini are applied to nodes A and B, respectively. Duringthe threshold voltage generation phase of the compensation period,signal EM is asserted and switch SW3 is turned on so that current flowsthrough drive transistor TD to charge up the capacitance at node B. Asthe voltage at node B increases, the current through drive transistor TDwill be reduced because the gate-source voltage Vgs of drive transistorTD will approach the threshold voltage Vt of drive transistor TD. Thevoltage at node B will therefore go to Vref−Vt.

After compensation (i.e., after initialization and threshold voltagegeneration), data is programmed into the compensated display pixels.During programming, emission transistor SW3 is turned off by deassertingsignal EM and a desired data voltage D is applied to node A using dataline 26. The voltage at node A after programming is display data voltageVdata. The voltage at node B rises because of coupling with node A. Inparticular, the voltage at node B is taken to Vref−Vt+(Vdata−Vref)*K,where K is equal to Cst1/(Cst1+Cst2+Coled), where Coled is thecapacitance associated with diode 30.

After compensation and programming operations have been completed, thedisplay driver circuitry of display 14 places the compensated andprogrammed display pixels into the emission mode (i.e., the emissionperiod is commenced). During emission, signal EM is asserted for eachcompensated and programmed display pixel to turn on transistor SW3. Thevoltage at node B goes to Voled, the voltage associated with diode 30.The voltage at node A goes to Vdata+Voled−(Vref−Vt)−(Vdata−Vref)*K. Thevalue of Vgs−Vt for the drive transistor is equal to the differencebetween the voltage Va of node A and the voltage Vb of node B. The valueof Va−Vb is (Vdata−Vref)*(1−K), which is independent of Vt. Accordingly,each display pixel 22 has been compensated for threshold voltagevariations so that the amount of light 40 that is emitted by each of thedisplay pixels 22 in the row is proportional only to the magnitude ofthe data signal D for each of those display pixels.

FIG. 4 is a cross-sectional side view of conventional OLED display pixelstructures. As shown in FIG. 4, the pixel structures are formed on aclear polyimide (PI) substrate 100. Multiple buffer layers 102 areformed on the PI substrate 100. Polysilicon 108 is patterned on bufferlayers 102 to form an active region for drive transistor 106. Gateinsulating layer 104 is formed on buffer layers 102 over polysilicon108. A metal gate conductor 110 is formed on gate insulating layer 104and serves as the gate terminal for drive transistor 106. A metal path130 that is formed adjacent to transistor 106 may serve as one of thescan lines for the display pixel. A silicon nitride passivation layer(not shown in FIG. 4) may be formed on gate insulating layer 104 overmetal structures 110 and 130.

Thin-film drive transistor 106 formed in this way passes current betweencathode 58 (i.e., an indium tin oxide electrode) and anode 116 (i.e., ametal layer) of light-emitting diode 119. As this current passes throughorganic light-emitting diode emissive electroluminescent layer (emissivelayer) 118, light 122 is generated. Display pixels generating light 122in this way is typically referred to as top emission display pixels.

During normal display operations, scan line 130 is sometimes biased to anegative voltage (i.e., scan line 130 can be biased to −5V). Assumingbuffer layers 102 includes two buffer layers, a negative charge isinduced at the top of the PI substrate 100. Negative charge induced inthis way can undesirably decrease the amount of current flowing throughdrive transistor 106 (i.e., the electric field generated between scanline 130 and the channel of transistor 106, as indicated by line 132,can negatively impact the performance of transistor 106). It maytherefore be desirable to form display pixels that are immune to thishorizontal field effect.

In accordance with an embodiment, a display pixel 22 having a bottomconductive shield is provided (see, e.g., FIG. 5). As shown in FIG. 5,thin-film transistor structures such as thin-film drive transistor TDmay be formed on a transparent substrate 200 made from as glass,polyimide, or other transparent dielectric material. Thin-filmtransistor TD may serve as the display pixel drive transistor TD that isdescribed in connection with FIG. 3.

One or more buffer layers such as buffer layers 306 may be formed onsubstrate 200. Buffer layers 306 may include layers sometimes referredto as a multi-buffer (MB) layer, an active oxide layer, and other layersformed from any suitable transparent dielectric material.

Active material 208 for transistor TD may be formed on buffer layers202. Active material 208 may be a layer of polysilicon, indium galliumzinc oxide, amorphous silicon, or other semiconducting material. A gateinsulating layer such as gate insulating layer 204 may be formed onbuffer layers 202 and over the active material. Gate insulator 204 maybe formed form a dielectric such as silicon oxide. A conductive gatestructure such as gate conductor 210 may be disposed over gatinginsulator 204. Gate conductor 210 may serve as the gate terminal forthin-film transistor TD. The portion of active material 208 directlybeneath gate 210 may serve as the channel region for transistor TD.

A conductive path such as path 230 may be formed in close proximity totransistor TD. Path 230 may, for example, be part of a control line forconveying one of the control/data signals to display pixel 22. In onearrangement, path 230 may be part of a scan line for carrying signalSCANT to corresponding switch SW1 in pixel 22 (FIG. 3). In anotherarrangement, path 230 may be part of a scan line for carrying signalSCAN2 to corresponding switch SW2 in pixel 22. In yet anotherarrangement, path 230 may be part of a control line for carrying signalEM to corresponding switch SW3 in pixel 22.

A passivation layer such as a silicon nitride layer (not shown in FIG.5) may optionally be formed on gate insulating layer 204 and over gate210. After deposition of the passivation layer, a hydrogenationannealing process may be applied to passivate the thin-film transistorstructures.

One or more dielectric layers 212 (sometimes referred to as interlayerdielectric or “ILD” layers) may be formed over the thin-film transistorstructures. The material with which gate 210 and path 230 is formed issometimes referred to as “M1” metal. The dielectric layer in which theM1 metal is formed may therefore be referred to as an M1 metal routinglayer.

Thin-film transistor structures such as thin-film transistor TD may passcurrent between cathode 220 (e.g., a transparent conductive layer suchas indium tin oxide or indium zinc oxide) and anode 216 (e.g., a lightreflecting metal layer) of light-emitting diode 219. As this currentpasses through organic light-emitting diode emissive electroluminescentlayer (emissive layer) 218, light may be generated. Light generated inthis way may pass through a corresponding color filter element (notshown), which imparts a desired color to the emitted light. In general,either top or bottom emission display pixel configurations can beimplemented for display 14.

As described above, electric field can sometimes be generated betweentransistor TD and an adjacent control path such as path 230, asindicated by dotted field line 232. In accordance with an embodiment ofthe present invention, a conductive shielding structure such as shield250 may be formed directly beneath drive transistor TD within bufferlayers 202. Shield 250 should not be in direct contact with activematerial 208 and gate insulating layer 204. Shielding structure 250 maybe formed from transparent conductive materials such as indium tinoxide, molybdenum, and molybdenum tungsten or opaque conductivematerials such as titanium, copper, aluminum, or other metals. Formed inthis way, conductive bottom shield 250 may serve to block any horizontalfield generated from metal path 230 or any other adjacent control linesfor transistor TD (e.g., shield 250 may prevent any undesired horizontalelectric fields from negatively impacting the operation of transistorTD). Shield 250 formed below transistor TD in this way is thereforesometimes referred to as a “bottom” shield or an electric field shield.

In general, it may only be desirable to form bottom conductive shieldsdirectly below the drive transistors in each pixel. In other words,bottom conductive shields need not be formed for the peripheralswitching transistors SW1, SW2, and SW3 (FIG. 3). Forming shield 250only under drive thin-film transistor TD can help reduce any undesiredparasitic capacitance within pixel 22, thereby minimizing dynamic powerconsumption.

The structures of FIG. 5 form a single subpixel of a particular color.There may be three or four subpixels per display pixel 22 or othersuitable number of subpixels per display pixel 22 in display 14. FIG. 6is a diagram of an exemplary display pixel 22 having three subpixels22-R, 22-G, and 22-B. Subpixel 22-R may include circuitry for displayingred light (e.g., subpixel 22-R may include a light-emitting diode thatemits light through a red color filter element). Subpixel 22-G mayinclude circuitry for displaying green light (e.g., subpixel 22-G mayinclude a light-emitting diode that emits light through a green colorfilter element). Subpixel 22-B may include circuitry for displaying bluelight (e.g., subpixel 22-B may include a light-emitting diode that emitslight through a blue color filter element). This is merely illustrative.In general, display pixel 22 may include any number of subpixelsconfigured to transmit red light, green light, blue light, cyan light,magenta light, yellow light, white light, and/or other types of light inthe visible spectrum.

As shown in FIG. 6, each of the subpixels includes a drive transistor TDand a respective conductive light shield 250 that directly overlaps withthe footprint of drive transistor TD. Configured in this way, lightshield structures 250 serve to prevent any electric field generated as aresult of bias voltages applied on control path 230 from interferingwith the operation of the drive transistors. The example of FIG. 6 inwhich bottom shields 250 are electrically floating (i.e., shields 250are not actively driven by any pull-up or pull-down circuits and are notconnected to one another) is merely illustrative. In other suitablearrangements, bottom shields 250 may be shorted using conductiveshorting path 252 (see, e.g., FIG. 7).

As shown in FIG. 7, conductive shorting path 252 may be formed in thesame layer as conductive shields 250 (e.g., conductive shorting path 252may be formed in buffer layers 202 of FIG. 5). Conductive shorting path252 may also be formed from the same material as that of shields 250(e.g., shorting path 252 may be formed from transparent conductivematerials such as indium tin oxide, molybdenum, and molybdenum tungstenor opaque conductive materials such as titanium, copper, aluminum, orother metals). Shorting the bottom shields together via conductive path252 can provide improved shielding capabilities, especially when paths252 are shorted to some power supply line.

FIG. 8 is a diagram showing an array of pixels 22 in display 14. Asshown in FIG. 8, at least a portion of bottom shields 250 (e.g.,conductive shields 250-R, 250-G, and 250-B) can be shorted to a powersupply line 254 (e.g., a power supply line on which ground power supplyvoltage V_(SSEL) is provided) via path 252. Bottom shield shorting paths252 may be coupled to ground line 254 only at the periphery of display14. Connected in this way, the bottom shield in each display subpixel isdriven to a constant voltage V_(SSEL), which enables the drivetransistor to operate in a more consistent manner across the entiredisplay pixel array.

Still referring to FIG. 8, the bottom shields in at least some displaypixels 22 are floating and are not connected to power supply line 254.This is merely illustrative. As another example, the conductive shields250 of each subpixel in the entire pixel array may be electricallyfloating. As yet another example, the conductive shields 250 of eachsubpixel in the entire pixel array may all be shorted to a ground powersupply line, a positive power supply line, or other power supply lines.

As described above in connection with FIG. 5, bottom shielding structure250 may be formed in buffer layers 202. In the arrangement in whichbottom shielding structure 250 is shorted to a ground line (e.g., acommon cathode electrode), the bottom shielding structure can be coupledto the cathode through conductive through-hole or “via” structuresformed through the thin-film transistor layers.

A cross-sectional side view of a peripheral portion 260 of display 14showing how the bottom shielding structure can be shorted to the cathodeelectrode is illustrated in FIG. 9. As shown in FIG. 9, conductiveshorting path 252 is formed in buffer layers 202 and may extend into theperiphery of display 14. One or more M1 metal routing paths such asmetal structure 231 may be formed on gate insulating layer 204. A firstvia structure 290 may be formed through layers 212 and 204 to form acontact with bottom conductive path 252. In particular, via 290 mayestablish an electrical connection between path 252 and anode 216. Asecond via structure 292 may be formed through layer 218 to form acontact with anode 216. In particular, via 282 may serve to establish anelectrical connection between anode 216 and cathode 220. Configured inthis way, bottom shielding structures 250 of the type shown in FIGS. 5,7, and 8 may be shorted to the grounding cathode electrode throughconductive path(s) 252 and vias 290 and 292.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display, comprising: a substrate; a thin-filmtransistor formed over the substrate; at least one buffer layerinterposed between the thin-film transistor and the substrate; and aconductive shielding structure formed in the buffer layer directly belowthe thin-film transistor, wherein the conductive shielding structure isformed from transparent conductive material.
 2. The display defined inclaim 1, wherein the conductive shielding structure is electricallyfloating.
 3. The display defined in claim 1, further comprising: a powersupply line, wherein the conductive shielding structure is shorted tothe power supply line.
 4. The display defined in claim 1, furthercomprising: a cathode electrode shorted to the conductive shieldingstructure through vias.
 5. The display defined in claim 1, wherein thethin-film transistor has a gate formed on a gate insulating layer, andwherein the conductive shielding structure is not in direct contact withthe gate insulating layer.
 6. The display defined in claim 1, furthercomprising: additional thin-film transistors, wherein the conductiveshielding structure is only formed below the thin-film transistor butnot below the additional thin-film transistors.
 7. A method ofmanufacturing a display pixel, comprising: forming a thin-filmtransistor over a substrate; forming a buffer layer interposed betweenthe thin-film transistor and the substrate, wherein the buffer layer hasfirst and second opposing surfaces; and forming an electric field shieldin the buffer layer for the thin-film transistor, wherein the electricfield shield is interposed between the first and second opposingsurfaces of the buffer layer.
 8. The method defined in claim 7, whereinforming the electric field shield comprises forming a conductiveshielding structure directly below the thin-film transistor.
 9. Themethod defined in claim 7, further comprising: forming a light-emittingdiode that is coupled to the thin-film transistor.
 10. The methoddefined in claim 9, wherein the light-emitting diode has a cathodeelectrode, the method further comprising: shorting the electric fieldshield to the cathode electrode through conductive via structures. 11.The method defined in claim 7, wherein the electric field shield is notactively driven.
 12. The method defined in claim 7, further comprising:forming an additional thin-film transistor over the substrate; andforming an additional electric field shield in the buffer layer for theadditional thin-film transistor.
 13. The method defined in claim 12,further comprising: forming a conductive path in the buffer layer thatshorts the electric field shield and the additional electric fieldshield.
 14. An electronic device display, comprising: display pixelsarranged in an array, wherein each display pixel in the array comprises:a drive transistor; and a conductive shield formed below the drivetransistor; and a conductive path that conveys signals to the displaypixels, wherein the conductive shield blocks the drive transistor fromelectric fields generated by the conductive path in at least one of thedisplay pixels in the array.
 15. The electronic device display definedin claim 14, where each display pixel in the array further comprises alight-emitting diode coupled to the drive transistor.
 16. The electronicdevice display defined in claim 15, wherein the conductive shield ineach display pixel in the array is electrically floating.
 17. Theelectronic device display defined in claim 15, wherein the conductiveshield in each display pixel in the array is shorted to a commonelectrode.
 18. The electronic device display defined in claim 15,wherein the conductive shield in each display pixel in a first portionof the array is electrically floating, and wherein the conductive shieldin each display pixel in a second portion of the array is shorted to acommon electrode.
 19. The method defined in claim 7, wherein forming thebuffer layer comprises forming a first buffer layer and a second bufferlayer, and wherein forming the electric field shield comprises formingthe electric field shield between the first and second buffer layers.